Slew Mode Control of Transient Phase Based on Output Voltage Slope of Multiphase DC-DC Power Converter

ABSTRACT

A multi-phase switch mode, voltage regulator has a transient mode portion in which a phase control output is coupled to one or more control inputs of one or more switch circuits that conduct inductor current through one or more transient phase inductors, from amongst a number of phase inductors. A slew mode control circuit detects a high slope and then a low slope in the feedback voltage and, in between detection of the high slope and the low slope, pulses the phase control output of the transient mode portion so that the switch circuit that conducts transient phase inductor current adds power to, or sinks power from, the power supply output. Other embodiments are also described.

This application is a Continuation-in-Part of co-pending U.S.application Ser. No. 15/419,259, filed Jan. 30, 2017, which claims thebenefit of the earlier filing date of co-pending U.S. Provisional PatentApplication No. 62/360,858, filed Jul. 11, 2016.

FIELD

An embodiment of the invention relates to switch mode multi-phase dc-dcvoltage regulator circuits used in portable consumer electronic devices.Other embodiments are also described.

BACKGROUND

Output voltage regulation and maintaining the accuracy of the regulatedvoltage provided by a switch mode power converter can be a verydemanding task. In the field of multi-function portable consumerelectronic devices (also referred to here as mobile devices, such assmartphones, tablet computers, and laptop computers) the powerrequirements of the constituent components such as the display screen,the wireless communications interface, the audio subsystem, and thesystem on a chip (SoC) or applications processor are quite demanding.For example, in such devices, the load on an output node of a voltageregulator can exhibit sudden changes that are so great, e.g. as fast as100 Amperes-1000 Amperes per microsecond, that the output node exhibitstransient voltage droop. Attempts at reducing the transient voltagedroop by the conventional approach of simply using larger loadcapacitance and higher performance decoupling and/or filter capacitorsmay not be practical in many instances.

Another solution to achieve fast transient recovery that has beensuggested is a transient recovery circuit that responds only to fastchanges in the load, so as to suppress both output voltage overshoot andundershoot (droop). The transient recovery circuit may operateindependently of a regular feedback circuit that controls the phases ofa multi-phase switching regulator more “slowly”, during steady stateoperation (when the load is not changing rapidly). The transientrecovery circuit overrides a control voltage from the regular feedbackcircuit so as to control the duty cycle of a pulse width modulationcircuit in the switching regulator, only during transient conditions.The transient recovery circuit may also control a dedicated phase, of amulti-phase switching regulator, and may remain inactive duringnon-transient conditions.

SUMMARY

An embodiment of the invention is a method for dc-dc power conversionthat uses a multi-phase switch mode power supply circuit that has atleast two controller portions, one of which is referred to here as atransient mode portion. The phases include one or more transient phasesthat are controlled by the transient mode portion, where the transientphases are controlled so that they do not transfer power to the powersupply output during a steady state load condition. The transient modeportion has a slew mode control circuit that detects a high slope andthen a low slope in a feedback voltage from the power supply output.Between the times the high slope and low slope are detected, a switchcircuit of the transient phase is pulsed so as to deliver a controlledamount of power to the power supply output (which reduces the voltagedroop that would otherwise have occurred during a step load transientcondition). The pulsing of the switch circuit may be controlled by acurrent limiter blocker or by an on-time blocker. The transient phaseinductor current thus rises to an upper level and then falls to a lowerlevel (as determined by the particular blocker being used), and thenrepeats, until the low slope is detected at which point the switchcircuit is turned off while allowing the inductor current to circulateand fall to zero. If no further high slope is then detected, then thetransient phase is disabled (e.g., both the high side and the low sideswitches of the switch circuit are turned off) once the inductor currentin the transient phase has reached zero. Because the transient phase isoperated in this manner only during transient conditions, the switchingloss that takes place in the switching transistors of the transientphase can be tolerated, even in an embodiment where the transient phasehas the smallest inductor amongst all of the phases (e.g. at least 100times smaller than the largest inductor in the other phases) and itsswitching frequency may be significantly greater than that of the otherphases. Simulations show that adding the transient mode portion resultsin significant reductions in droop.

Another embodiment may be to enter the transient phase switching regime(also referred to here as slew mode control of the transient phases)only when the output voltage has dropped significantly (as detected by avoltage comparator against a predetermined voltage threshold, Vunder)and together with the earlier mentioned voltage slope detector. In otherwords, the transient phase switching is initially enabled only if i) theoutput voltage has dropped a certain amount (e.g., below a predeterminedabsolute voltage threshold) and ii) the high slope is detected. TheVunder detection may also be used after the initial triggering oftransient phase switching, so that the transient phase is not turned ONeach time the high slope is detected unless the output voltage is alsodetected to be below Vunder. Such a scheme may prevent triggering of thetransient phase switching at light load operations.

A similar benefit is also expected when the transient mode portion isconfigured to mitigate an overshoot, by pulsing the transient phase toobtain a controlled sink of power from the power supply output (whichreduces the voltage overshoot that would otherwise have occurred duringa step load transient condition).

In one aspect, a step-down, DC-DC switch mode power converter is dividedinto two portions, a steady state mode portion which serves to provideprimarily the steady state or slow varying portion of the load current,e.g., the maximum rated DC load current can be sourced by the steadystate phases, and a transient mode portion which is only enabled whenthe power supply output is in a transient condition (it is disabled whenthe power supply output is in a steady state condition). The transientmode portion may have a slew mode control circuit control the transientphase, independent of the steady state portion but responding to thesame feedback voltage.

In a current limiter blocker embodiment, the transient phase's pulsewidth is controlled on a per-cycle (repeating) basis, based on sensingthe inductor current through the transient phase and comparing thesensed current to an inductor current limit or threshold, whichthreshold is defined by at least two programmable values. Theseprogrammable values result in the inductor current limit being selectedto be high when a detected, feedback voltage change is large (e.g.,large droop, or large overshoot), and low when the detected feedbackvoltage change is small. Also, the off-time (the time interval duringwhich the switch circuit is turned OFF and the inductor current isallowed to circulate and fall) should be long enough to restrictrun-away of the inductor current. It may be made programmable based on alook-up table that lists different combinations of input voltage andoutput voltage states.

In an on-time blocker embodiment, the transient phase's pulse width isset to a predetermined time interval, without relying on sensing theinductor current. This is referred to here as on-time (the intervalduring which the switch circuit is turned ON), and is selected to belong when the output voltage change is large, short when the outputvoltage change is small. This approach is preferred vs. the currentlimiter blocker, when the transient phase inductor is relatively small(e.g., 10 nH and smaller), because the inductor current in that caserises/falls too quickly for the reaction time of the current limiterblocker circuitry. The off-time here should be long enough to allow theinductor current to fall enough so that the next time the phase isturned ON, there is a desired time interval over which the inductorcurrent then ramps back up to its upper level and also to restrictinductor current run-away.

The transient phase switch circuit or power-stage may be synchronous(with both a high-side transistor switch and low-side transistor switch)or it may be asynchronous with the low-side transistor replaced oraugmented with a free-wheeling diode in which case the slew modecontroller may just need to control the high side switch.

The supply (power supply input) of the transient phase may be the sameor different from that of the steady-state phase. A separate lowersupply voltage may be used for the transient phase to enable the use offaster and smaller transistors whose fabrication process technology mayallow them to be implemented directly in the same fabrication process asthe load itself.

The above summary does not include an exhaustive list of all aspects ofthe present invention. It is contemplated that the invention includesall systems and methods that can be practiced from all suitablecombinations of the various aspects summarized above, as well as thosedisclosed in the Detailed Description below and particularly pointed outin the claims filed with the application. Such combinations haveparticular advantages not specifically recited in the above summary.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention are illustrated by way of example andnot by way of limitation in the figures of the accompanying drawings inwhich like references indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment of the invention in thisdisclosure are not necessarily to the same embodiment, and they mean atleast one. Also, in the interest of conciseness and reducing the totalnumber of figures, a given figure may be used to illustrate the featuresof more than one embodiment of the invention, and not all elements inthe figure may be required for a given embodiment.

FIG. 1 is a combined block diagram and circuit schematic illustrating anexample switch mode power supply circuit supplying power to a load.

FIG. 2 is a graph of several waveforms illustrating droop improvement.

FIG. 3 is a graph of waveforms representing transient phase and otherphase inductor currents during a transient load condition, and waveformsof several constituent voltage signals and control signals of amulti-phase switch mode power supply circuit having a transient modeportion controlling a transient phase, and another portion controllingother phases.

FIG. 4 is a combined block diagram and circuit schematic of a slew modecontrol circuit that uses a current limit blocker to control pulse widthof the phase control output.

FIG. 5 is a graph of waveforms obtained from a simulation of the slewmode control circuit of FIG. 4.

FIG. 6 is a circuit schematic illustrating a slew mode control circuitthat uses an on-time blocker to set the pulse width of the phase controloutput.

FIG. 7 illustrates an example of a finite state machine used to generatethe tri-state and pulsed output signals of the slew mode controlcircuit.

FIG. 8 is a graph of output impedance vs. frequency of an embodiment ofthe switch mode power supply circuit.

FIG. 9 is a circuit schematic illustrating the 4 phases of FIG. 1 in anembodiment where the transient phase inductor L4 is a coupled inductorco-packaged with inductor L3.

DETAILED DESCRIPTION

Several embodiments of the invention with reference to the appendeddrawings are now explained. Whenever the shapes, relative positions andother aspects of the parts described in the embodiments are notexplicitly defined, the scope of the invention is not limited only tothe parts shown, which are meant merely for the purpose of illustration.Also, while numerous details are set forth, it is understood that someembodiments of the invention may be practiced without these details. Inother instances, well-known circuits, structures, and techniques havenot been shown in detail so as not to obscure the understanding of thisdescription.

FIG. 1 is a combined block diagram and circuit schematic illustrating anexample, multi-phase switch mode power supply (SMPS) circuit 2 supplyingpower to a load. The multi-phase SMPS circuit 2 is for controlling thetransfer of power to a power supply output 3, to which a filtercapacitor 3 and the load are coupled as shown, through a number ofphases having phase inductors L1, L2, . . . respectively, where in thisexample four phases are shown coupled in parallel between a power supplyvoltage input Vin and the output 3. Note that the Li label is used hereas referring to not just an inductor but also the phase, which theinductor and a respective switch circuit SWi are a part. The multi-phaseSMPS circuit 2 may serve to regulate voltage at the output 3, forexample as a buck converter that steps down from a higher voltage powersupply input, Vin, to a lower voltage at the output 3. Just as anexample, Vin may be a battery voltage provided for example by arechargeable battery (e.g., having lithium based chemistry that producesa nominal battery voltage of 3.7 Volts), while the voltage at the output3 may be 1 Volt dc. In the case where the multi-phase SMPS circuit 2 isimplemented as part of a portable consumer electronics device (e.g., asmart phone, a tablet computer, a laptop computer), in which the onlysource of power is a rechargeable battery, the constituent circuitcomponents of the SMPS circuit 2 such, described below, may also bepowered by the same Vin (or battery voltage).

In one embodiment, the SMPS circuit 2 has a steady state mode portion 4and a transient mode portion 5. The steady state mode portion 4 has oneor more phase control outputs that are coupled to one or more controlinputs of one or more switch circuits, respectively, that conductinductor current through one or more steady state phase inductors. Inthis example, there are a total of four phases where each phase Li has arespective inductor, of which phases L1, L2, and L3 are steady statephases, while L4 is a transient phase. The steady state mode portion 4has a low frequency or slow control loop (referred to here as having adc gain) that regulates the voltage on the output 3, based on a feedbackvoltage Vfb, by operating the steady state phases and not the transientphases. The steady state mode portion 4 may implement any suitable powercontroller algorithm for controlling the steady state phases, such asvoltage mode, current mode, hysteretic or on/off time controllers, andmay be optimized for static efficiency at light and large load, dcregulation, line transients and electro-magnetic interferencecompliance. The concepts here are more generally applicable to one ormore other phases and one or more transient phases, as the number oftransient and other phases may be different than illustrated in thefigures.

The inductor current in each phase is conducted through its respectiveswitch circuit SWi, which in this example includes a high side solidstate switch (e.g., a switching transistor) and a low side solid stateswitch, where the high side switch is tied to the power supply inputVin, while the low side switch is tied to the circuit or system ground.In this case, each switch circuit SWi has two control inputs, one forthe high side switch and one for the low side switch. An alternative tohaving both a high side and a low side switch is to simply use the bodydiode of a transistor that is kept turned OFF, or to replace the lowside transistor with a stand alone diode, in both cases resulting inonly one control input for the switch circuit SWi. Also, if desired, oneor more of the phases may have its switch circuit SWi coupled to adifferent power supply input voltage. For example, in the case of theportable consumer electronics device, the steady state phases L1-L3 maybe coupled to the battery voltage, while the transient phase L4 iscoupled to a lower power supply voltage, for example through the use ofan additional power converter (e.g., another buck converter or acapacitor divider from the battery) that steps down the 3.7 Voltsbattery voltage to just 1 Volt or any voltage that is lower than thebattery which is required to provide the necessary regulated outputvoltage at the load. One clear benefit of having the supply for thetransient phase connected to a supply other than the battery is toprevent the transient phase from demanding fast, transient spikes ofcurrent directly from the battery. This may be desirable if theconstituent transistors of the SW4 switch circuit, in phase L4, arecomposed of smaller transistors that have a lower voltage rating thanthe devices that form the switch circuits of the steady state phasesL1-L3. Such smaller or lower voltage transistors offer lower switchinglosses and/or higher switching speeds, which are beneficial at highswitching frequencies (e.g., above 30 MHz) which may be needed tocontrol a transient phase inductor having an inductance of 10 nH orless. In one embodiment, the smaller or lower voltage transistors arefabricated using the same microelectronic fabrication process that isused for making the larger transistors that form the switches of thesteady state phases.

As explained below, the inductor of the transient phase L4 may have muchsmaller inductance than every one of the inductors of the steady statephases L1-L3, so as to better support the role of the transient phase L4in providing the rapid power boost (or sink) needed for a powerconverter to recover more quickly from a transient load. As an example,each of the one or more transient phase inductors (here, just L4) mayhave at least 10 times smaller inductance than every one of the steadystate phase inductors (here, L1-L3). As another example, each of the oneor more transient phase inductors may have at least 100 times smallerinductance than the steady state phase inductor that has the largestinductance. An example of the latter combination may be L1=1 microHenry,L2=0.47 microHenry, L3=0.1 microHenry, and L4=0.01 microHenry (althoughit is not necessary that all of the other phases, L1-L3 have differentinductance values). The transient phase inductance may be so small thatit could be implemented as a single wire segment or partial loop (nomultiple winding or coil like structure as in a discrete inductor). Thatwire segment or partial loop directly connects the power supply to itstransient phase switching transistor (so that there is no discreteinductor in the transient phase). The inductance of the wire segment orpartial loop may be increased, by designing for some magnetic couplingwith a nearby discrete inductor of a steady state phase (also referredto as a coupled inductor). As seen in FIG. 9 for example, the transientphase inductor (here, L4) may be the result of a coupled inductor designor a stray/parasitic effect, where there is magnetic coupling between awinding and a separate partial loop of wire: this produces theinductance of the steady state phase inductor L3 at terminals a, b, andthe inductance of the transient phase inductor L4 at terminals a, c inthe same passive component package.

Inductance levels of 0.01 microHenry and less are so small that atypical buck converter design for using them as the steady state phaseinductors may need switching transistors that need be operated on theorder of 100 MHz switching frequency. If used for steady stateoperation, such small inductors may also require switching transistorswhose switching losses are relatively small, in order to limit the totalswitching loss of the converter to a reasonable level. That means small,low voltage transistors are needed. These however cannot be operated at3.7 Volts (the typical lithium based rechargeable battery voltage in aportable consumer electronic device), and so an additional regulator isneeded to produce the lower, input supply voltage.

In accordance with an embodiment of the invention, the transistorswitching losses associated with such a small inductor is tolerable,even when the inductor is used in a transient phase that may have thesame specification for its switching transistors as the ones used forthe steady state phases, which can withstand full battery voltage. Theswitching losses are kept to a low enough level, because of therelatively short amount of time that the transient phase is enabled.Using larger transistors that can operate at full battery voltage alsoavoids the need for a separate step down regulator. A further benefit isreduced expense when all of the switching transistors, for all of thephases, are formed in the same monolithic integrated circuit as thesteady state and transient mode portions 4, 5. Further, in theembodiments where the transient phase is not turned ON unless, inaddition to the high slope, an absolute output voltage threshold is alsodetected, which will henceforth be called Vunder (for undershoot ofvoltage below a threshold) and Vover (for overshoot of voltage above athreshold), then transient phase operation is restricted to situationsthat are only within a bandpass detection of the output voltage slope.This may reduce power loss at light load operation.

The transient mode portion 5 has an input to receive the feedbackvoltage Vfb from the power supply output 3. The feedback voltage may beobtained through a low impedance path directly from the power supplyoutput 3, or it may be a conditioned, filtered or other derived form ofthe voltage on the power supply output 3. A slew mode control circuit(not shown in FIG. 1 but described below using examples in FIG. 4 andFIG. 6) is configured to detect a high slope and then a low slope in thefeedback voltage, and in response operate the switch circuit of thetransient phase inductor L4 (through the phase control output). In FIG.1, there are two, phase control outputs shown for each phase, because ofthe type of control needed in the case where the switch circuit SWi iscomposed of both a high side and a low side switch. When the switchcircuit SWi has only a high side switch and a diode tied to ground, asingle, phase control output for each phase is sufficient.

The embodiments of the invention described here have shown reductions inthe droop exhibited by the voltage on the output 3 during loadtransients, as for example depicted in the graph of FIG. 2. The stimulushere is a load step (current) that increases abruptly at time marker 50,causing a droop of over 100 millivolts in the case of a regulated outputvoltage of 1 Volt. It can be seen that with the addition of thetransient mode portion 5 including the particular slew mode controlcircuit described below, operating upon an additional phase L4, thedroop can be reduced between 43% and 75%, e.g., 30 millivolts in oneinstance, and even further by almost 50 millivolts (if a higher dc gainis programmed into the steady state mode portion 4—see FIG. 1). Therelative improvements show here are as compared to the case where only 3phases are operating (as controlled by the steady state portion 4). Notethat the steady state portion 4 may be configured to implement anysuitable switch mode power supply controller topology, for example amulti-phase synchronous buck converter topology which may also use thefeedback voltage Vfb in implementing a voltage regulation control loopto regulate the dc output voltage of the power supply output 3.

In other words, the simulation results in FIG. 2 show that if a fastresponding transient phase is added and is allowed to operate onlyduring intervals between a high slow and a subsequent low slope in thevoltage at the output 3, then a faster dc regulation loop may bedesigned into the steady state mode portion 4, resulting in a smallervoltage drop during the high load condition. Such a solution may alsoallow a reduction of the capacitance area in the filter capacitor C (seeFIG. 1), where it is understood that the filter capacitor C may serve toimprove dc stability of the steady state mode portion while filteringout the voltage ripple that is produced by a switch mode power supply.Viewed another way, the transient mode portion 5 along with the additionof the transient phase L4 is enabled only until dc regulation of theoutput 3 is achieved by the steady state mode portion 4.

Referring now to FIG. 3, relevant waveforms produced by a simulation ofthe system in FIG. 1 are shown. There are four rows of waveforms shown.The first row shows two digital or bistable control signals that, asdescribed further below, may be used to produce the phase control outputwhich in turn may directly control the switch circuit SW4 of thetransient phase. The second row of waveforms illustrates the behavior ofthe feedback voltage Vfb. The third row shows the particular load stepthat is being used in this simulation, and the behavior of the transientphase inductor current. Finally, the fourth row contains the waveformsof inductor current in two of the steady state phases, respectively.

Beginning with prior to the time marker 50, the feedback voltage isessentially at a dc level dictated by the regulated output voltage.There are two, phase control outputs produced by the transient modeportion in this case, namely tristate and L4pwm—see FIG. 1. The L4pwm isderived from an intermediate control signal L4en depicted in FIG. 3. Thetristate signal is in its asserted state, which means that the transientphase is disabled (its high side and low side switches are turned OFF,and the inductor current through the transient phase is zero). The othercontrol signal L4en, prior to time marker 50, is de-asserted, whichmeans that L4pwm is not being pulsed.

Next, at time marker 50, a load step is applied and in response thevoltage on the output 3 drops as shown in the second row of waveforms.As highlighted by the small circles, and in particular the first circle,the feedback voltage drops fast enough such that its slope is detectedto be a high slope, in response to which tristate is de-asserted andL4en is asserted. This means that the phase control output is nowasserted so as to turn ON the switch circuit SW4 of the transient phase,resulting in a rapid increase in the transient phase inductor current asshown. Note how at this point, the inductor currents of the steady statephases have hardly begun increasing, consistent with the “slow” responseof the steady state mode portion. Here it should be noted that becausethe transient phase inductor L4 is relatively small (low inductance) itsinductor current will increase rapidly and needs to be controlled so asnot to short circuit the power supply input Vin. In one embodiment, therising inductor current is reversed and begins to fall, when apredefined current limit is reached. This is also referred to as thecurrent limit blocker version, which is described and shown in furtherdetail below (in connection with FIG. 4). After reaching the currentlimit, the high side switch of the switch circuit SW4 is turned OFFwhile allowing the inductor current to recirculate and ramp down throughthe low-side switch being turned ON, in one embodiment for a fixedoff-time interval (which as described below may be programmable).

At the end of the off-time interval, the high side switch of SW4 isagain turned ON which causes the transient phase inductor current toincrease again. This pulsed or open loop, bang-bang control of thetransient phase inductor current exhibits a generally saw tooth type ofwaveform as shown in FIG. 3. This behavior continues so long as the highslope is being detected on Vfb, but stops when the low slope isdetected. The reduction in slope of Vfb is being mainly caused by thepower boost from the transient phase and also as the current from restof the steady state phases catch up. Now, when the low slope isdetected, this causes the L4en signal to be de-asserted as shown (whiletristate continues to remain de-asserted). The de-assertion of L4encauses the phase control output that is directly driving the switchcircuit 4 (see FIG. 1) to be de-asserted so as to turn OFF the high sideswitch of switch circuit SW4, while allowing the inductor current in thetransient phase to circulate downward towards zero through the low sideswitch being turned ON. Note however that if a diode is used, instead ofturning ON the low side switch, it may automatically commutate thecurrent to zero and hence no extra control of the low-side transistor isneeded. Since the inductor current in the transient phase is rampingdown, the output voltage slope again starts increasing since the currentin the rest of the phase currents have yet to reach the loadrequirement. The inductor current reaches zero and may then stay at zerobefore the next high slope detection is detected which causes the phasecontrol output to be asserted again, which turns ON the high side of SW4thereby resulting in the inductor current ramping upward again, untilthe switch circuit SW4 is turned OFF due to either a current limit beingreached or a predetermined on-time interval expiring. The pulsed orsawtooth type behavior of the inductor current continues once againuntil the low slope is detected in Vfb, resulting in SW4 being turnedOFF and the inductor current being allowed to circulate towards zerothrough the low side switch being turned ON (or through a diode). Notehere however that in this second instance where the transient phaseinductor current crosses zero, the inductor currents of the steady statephases ramped upward considerably and are therefore contributing to theload, much more than at time marker 50. Therefore, the duration overwhich the transient phase inductor current is being pulsed (bang-banginterval) has become smaller.

Next, at time marker 50.7, the high slope is again detected (for thethird time), which causes tristate to be de-asserted (it had beentemporarily asserted when the transient phase current had reached zeroand until the next high slope was detected). This also causes L4en to beasserted, resulting in the transient phase inductor current rampingupward for the third time (from zero). The inductor current reaches itsupper level (again either due to a current limit blocker being activatedor due to an on-time interval expiring) and L4en being earlierde-asserted due to slower slope detection, turns OFF the high side ofSW4 and allows the inductor current to ramp down to zero through thelow-side switch/diode. After this point, since there are no furtherdetections of the high slope, the phase control output is no longerpulsed and so the inductor current is allowed to continue to fall andreach zero, and then remains at zero because the slope in Vfb is nothigh enough (no further high slope is detected). The latter circumstanceagain causes tristate to be asserted, this time signifying that switchcircuit SW4 is disabled since the inductor current is now also zero. Itcan also be seen that starting at about time marker 51 and beyond, theinductor currents of the steady state phases have reached a high enoughlevel that enables dc regulation of the output voltage to be stable (byoperation of the steady state mode portion 4).

FIG. 3 also shows how a transient load condition may be defined, asstarting with the step load current at time marker 50, and ending whenthe transient phase inductor current has reached zero and the powersupply output (or the feedback voltage from the power supply output) hasreturned to within a nominal specified voltage ripple, for instance attime marker 51.3. Thereafter the steady state load condition resumes,because the power supply output or the feedback voltage is within thenominal specified voltage ripple.

Another view of the high slope and low slope-based slew mode controlcircuit is to consider the behavior of the output impedance at the powersupply output 3. Under this view, the switching operation of thetransient phase may be triggered between the steady-state bandwidth ofthe other phases and up to the bandwidth of any power supply decouplingor filter capacitors (e.g., some of which may be on-chip as part of theload, e.g., a system on a chip, SoC). This allows any transient droop(or overshoot, if so configured) to be reduced, to the output impedancecurve under large signal operation. Referring to FIG. 8, an example maybe from 0.3 MHz to 100 MHz where the steady state bandwidth of the powersupply circuit may be around 0.3 MHz while the filter capacitors may bedesigned to filter at up to 100 MHz using on-chip decoupling capacitors.The high and low slopes may also be programmed by considering the outputimpedance behavior in FIG. 8, where detection of the low slope may benear the bandwidth of the steady state phases, while the detection ofthe high slope is enabled by programming the cutoff frequency of LPF1 tobe near the bandwidth of the on-chip (load-side decoupling capacitors),by proper programming of the LPF1 and LPF2 filters and Voffhigh andVofflow offsets of the comparators. The entry and exit of the slew modecontrol of the transient phases may be based on high and low slopedetection using low pass filters (LPFs), offsets and comparator logicsas shown in FIG. 4 which is one example of its implementation. Otherexamples with different filter designs (with fewer or more filters),comparator logics and digital logic gates, may also be used to implementthe slew mode control.

FIG. 4 is a combined block diagram and circuit schematic of a slew modecontrol circuit. This figure serves to illustrate several aspects of theslew mode control circuit (that is part of the transient mode portion5), namely the addition of a non-overlapping control driver circuit atthe two phase control outputs (tristate and L4pwm), and a current limitblocker which is one implementation of a bang-bang control of thetransient phase inductor current during the time interval between thedetection of a high slope and a subsequent detection of a low slope. Aspointed out above however, not all elements or components shown in afigure are necessary or needed for a given embodiment. For example, thefurther phase control output, tristate, may not be needed in instanceswhere the switch circuit SW4 does not have an operating low sidetransistor (and instead relies on only a diode connected to ground, toprovide a one way recirculation path to ground). Also in thatembodiment, the non-overlap control driver circuit itself may not beneeded, since there is only the high side switch that needs to becontrolled, whereas in the case of both a high side and a low sidetransistor being used as shown, shoot through is a concern which ismitigated by the driver circuit ensuring that the timing between thecontrol signals at the gates of the high side and low side transistors,respectively, do not overlap so as to result in a shoot throughsituation.

The transient mode portion 5 depicted in FIG. 4 has a slew mode controlcircuit that is implemented in analog form, as follows. There is a firstor upper analog comparator whose output is a first bistable comparisonindication, slope H_N (where N indicates active low), between a firstlow pass filtered and offset version of the feedback voltage (V1) at a+ve input, and a second low pass filtered version of the feedbackvoltage (V2) at a −ve input. There is also a second (lower) analogcomparator whose output is a second bistable comparison indication,slope L_N (active low), between V2 at its +ve input and a third offsetversion of the feedback voltage (V3) at its −ve input. There is alsologic circuitry that produces the phase control output, L4pwm, to be afunction of the first and second comparison indications, slope H_N andslope L_N, where such logic circuitry includes the NOR gate, the ANDgate, the SR latch, and the finite state machine (FSM). An example ofthe FSM is given in FIG. 7.

Detailed operation of the circuit in FIG. 4 may be described as follows,while also referring to the waveforms and timing illustrated in FIG. 5.When the output voltage (voltage of the power supply output 3) is atsteady state as reflected in the feedback voltage Vfb, namely beforetime marker 0.6, Vfb and V2 are at the same voltage while V1 which is atthe +ve input of the upper comparator is parked at a predeterminedoffset, Voffhigh, in this example 5 millivolts above the −ve input ofthe upper comparator. As a result, the output of the upper comparator,slope H_N (active low), is initially high. Similarly, the initial stateof the lower comparator is also initially high since its +ve input ishigher, by the predetermined offset Vofflow (e.g. 3 millivolts), thanits −ve input. When a load transient occurs (in FIG. 5, at time marker0.5), Vfb (a filtered version of which is shown) begins to fall. Also,V2 begins to fall but since it is a low pass filtered version (asdefined by the transfer function of LPF2) it falls more slowly than Vfb.In addition, V2 also falls more slowly than V1 and V2, due to the lowercut off frequency of LPF2 relative to that of LPF1, where the lattergoverns the fall rate of both V1 and V3. As a result, when V3 crossesbelow V2 and then when V1 crosses below V2, the SR latch is set (L4engoes high) which may be described as the transient phase being enabled.The latter means that the transient phase is now operating in bang-bangcontrol, based on either current limiting or fixed on/off time control,which takes place, as seen in FIG. 5, between time marker 0.64 and timemarker 0.76. At time marker 0.74, the slope of Vfb is sensed when V2 hascontinued its decrease and crosses under both V1 and V3, where thiscondition causes both the upper and lower comparators, that is theiroutputs, to go high, so that the SR latch is now reset or cleared (L4engoes low). As seen in FIG. 7, this causes the FSM to change state at thenegative edge of L4en, which transitions SW4 from its high sidetransistor being turned ON to the high side transistor being turned OFFand the low side transistor being turned ON (achieved using in this caseboth phase control outputs, tristate and L4pwm). As explained above, ifthe switch circuit SW4 is of asynchronous design, meaning that only adiode connected to ground instead of a low side transistor, then the L4inductor current will recirculate towards zero automatically when thehigh side is turned OFF (so that the additional phase control output,tristate, may not be needed in that case).

As has been described above, FIG. 4 shows an example slew mode controlcircuit that uses a current limit-based blocker circuit that isconfigured to govern pulse width at the phase control output L4pwm, in abang-bang control scheme. This is based on i) a programmable value thatrepresents a desired limit on the transient phase inductor current, andii) another programmable value, e.g., Vsel, that represents a thresholdfor generating an “output error” or difference between the feedbackvoltage Vfb and Vsel. The pulse width is governed ultimately by the FSM(see FIG. 7), and this is governed on a per cycle or repeating basisbased on a circuit (not shown) that is sensing the transient phaseinductor current (IL4sens). This sensed current is compared to aninductor current limit or threshold, here at the −ve input of acomparator whose +ve input receives the sense inductor current, IL4sens.The threshold at the negative input may be defined by a Gm-controlledvalue, i.e., a value that is based on the output voltage error relativeto a set threshold, Vsel, and gained by an amplifier circuit, but islimited to be between a low limit and a high limit (both of which may beprogrammable). The inductor current limit is automatically generated tobe between the high limit and the low limit depending on the outputerror, and closer to the high limit when the error is large (e.g., Vfbhas drooped too much below Vsel), and closer to the low limit when theerror is small. This is also referred to here as a Gm-clamp. In otherwords, the limit on the transient phase inductor current that is reachedduring the time interval, referring now to FIG. 5, between time marker0.64 and 0.76 is decided by the change or error in the voltage of thepower supply output 3 (as reflected in the feedback voltage Vfb). Thismechanism helps prevent overshooting of the transient phase inductorcurrent, as a function of the present load, which is indirectly relatedto the change or error in Vfb or the output voltage. Still referring toFIG. 5, the time interval or off-time that is forced following the limitbeing reached by the inductor current may be programmable, and in thisexample is set to 20 nanoseconds by a delay cell as shown in FIG. 4,where this may be set to allow enough time so as to prevent a runaway ofthe inductor current. This off-time may be programmable, based on aprogrammable look-up table that lists a number of different power supplyinput and power supply output voltage conditions and their respectiveoff-times.

The pulse width of the phase control output L4pwm may alternatively begoverned without having to sense the inductor current of the transientphase. This embodiment of the slew mode control circuit is depicted byan example in FIG. 6, which uses what is referred to here as an on-timeblocker circuit. The on-time blocker automatically sets the pulse widthof L4pwm to a controllable value (representing a time interval), betweenan upper limit (long) and a lower limit (short), as a function of thepresent output error, and using an output voltage error-basedGm-controlled circuit that sets a delay cell to have the upper and lowerlimits. The on-time blocker thus also features the Gm-clamp, which meansthat the on-time is automatically selected to be a long interval whenthe output voltage change is large (e.g., when Vfb has drooped too farbelow Vsel), and short when the output voltage error is small. Theon-time interval in this case, referring to FIG. 5, is the time intervalduring which the inductor current is allowed to increase before it isautomatically forced to changed direction, without having to sense theinductor current. This approach of not having to sense the inductorcurrent is preferred, as opposed to the current limit blocker approachof FIG. 4, when the transient phase inductance is so small that theinductor current rises or falls too quickly for the reaction time of thecurrent limit blocker circuitry that produces the control signal ilimdet(current limit detected). In particular, if the L4 inductor is anair-core inductor then saturation may not be important such that thecurrent limit blocker approach described above may not be needed.

Still referring to FIG. 6 and the on-time blocker, a programmableon-time delay cell may be provided that represents the on-time interval.A further programmable value may be provided that represents theoff-time of the phase control output, for the on-time blocker embodimentof FIG. 6, where this off-time interval should be long enough to allowthe inductor current to fall enough so that next time the transientphase is turned ON, there is a desired time interval over which theinductor current can then ramp back up to its upper level and not toallow run-away. The off-time may be programmable based on a look-uptable that lists a number of different input voltage and output voltagestates and the respective off-times to be selected for each.

The SMPS circuit 2 may have additional programmable features that enableit to operate a transient phase inductance that is variable, e.g., inthe range of 1-10 nanoHenrys, or that enable it to be tuned for improvedefficiency at a given transient phase inductance. In particular, one ormore of the following features shown in FIG. 4 and FIG. 6 may be madeprogrammable via externally accessible registers: delay cells, the LPF1and LPF2, Voffhigh, Vofflow, Vsel, output voltage-controllable inductorlimits, output voltage controllable on-time with lower and upper limitscontrolling short and long on-time intervals and programmable off-time.

In one embodiment, the multi-phase SMPS circuit 2 may be divided intojust two portions, namely the state mode portion 4, which serves toprovide primarily the steady state or slow varying portion of thecurrent drawn by the load (from the power supply output 3), for examplethe maximum rated dc load current can be sourced by the steady statephases, and the transient mode portion 5 which is only enabled when thepower supply output 3 is in a transient load condition (e.g., it isdisabled when the power supply output 3 is in a steady state condition).Further, an output voltage based threshold called Vunder (forundershoot) and Vover (for overshoot) may be used in conjunction withthe slope detection mechanism, to control the initial triggering of thetransient phase switching.

Under certain oscillating load transient cases, where the load currentgoes from low to high and from high to low at a certain frequencywindow, e.g., between 10 MHz to 20 MHz, the transient phase may bepulling a certain fraction of DC load (some portion of the DC loadcurrent is being steered through the transient phase instead of thesteady state phases.) Such usage of the transient phase may beundesirable. To mitigate such a situation, a watch dog timer may be setto count down a time interval that is commensurate with the controlbandwidth of one or more of the steady state phases, e.g., between 1microsecond to 2 microseconds. The watch dog timer is reset by a steadystate phase ON control signal or pulse width modulation (PWM) controlsignal, that is controlling turn ON of one or more of the steady statephases. If the watch dog timer expires (because it was not timely resetby the control signal), then this forces the steady phases to turn ONand thereby ramp up their currents, up to a steady state current, e.g. acurrent that is set in accordance with a Gm-controlled value, such asdescribed above. In addition, this expiration of the watch dog timer mayforce the transient phase to be turned OFF. This advantageously preventsthe transient phase from having to bear a significant portion of thefull load. Furthermore, the amount of DC load that is steered away intothe transient phase may be shaped by design choices of theimplementation parameters so that the transient phase inductor isoptimized.

The following further statements of invention can be made.

A method for DC-DC power conversion that uses a multi-phase switch modepower supply circuit may include the following operations (some of whichmay partially overlap another in time). During a steady state loadcondition on a power supply output, one or more switch circuits thattransfer power to the power supply output through one or more otherphase inductors, respectively, that are a subset of a plurality of phaseinductors, are controlled so as to regulate voltage at the power supplyoutput. The steady state load condition may be defined as when the powersupply output is within a nominal specified voltage ripple. During atransient load condition on the power supply output, one or more switchcircuits are controlled to transfer power to the power supply outputthrough one or more transient phase inductors, respectively, that are adifferent subset of the plurality of phase inductors. The one or moreswitch circuits are controlled so that the one or more transient phaseinductors do not transfer power to the power supply output during thesteady state load condition. The transient load condition may be definedas starting with a step load current on the power supply output, andending when the inductor current has reached zero during ii) below, andthe power supply output has returned to within a nominal specifiedvoltage ripple. During the transient load condition,

-   -   i) the switch circuit for the transient phase inductor is pulsed        so as to increase the inductor current, when slope of the power        supply becomes high, and then    -   ii) the switch circuit is turned OFF such that the inductor        current decreases to zero, when the slope of the power supply        output becomes low, and    -   iii) i) and ii) are repeated, whenever the slope of the power        supply output becomes high and the inductor current reaches zero        in ii).

In another embodiment, the switching of the transient phase is triggeredat a rate that is between steady-state bandwidth of the other phases andup to the bandwidth of a load-side, on-chip passive which may include acapacitor. This may result in reduction of transient droop or overshootin accordance with an output impedance curve under large signaloperation, an example of which may be from 0.3 MHz to 100 MHz where thesteady state bandwidth of the power supply circuit may be around 0.3 MHzand the filter capacitors on the power supply output may be designed toattenuate above 100 MHz using load-side on-chip passives (e.g.,capacitors).

In accordance with yet another embodiment, a slew mode control circuitdetects a high slope and then a low slope in a feedback voltage, whichmay be the power supply output voltage itself or a version derivedtherefrom. This detection may also be referred to here as bandpass slopedetection. Such detection is implemented digitally by quantizing thefeedback voltage and an output voltage error, and detecting the slopesdirectly in the quantized feedback voltage.

While certain embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat the invention is not limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those of ordinary skill in the art. For example, while FIG. 4and FIG. 6 depict analog solutions to various aspects of the slew modecontrol circuit, including the detection of the high and low slopes bythe upper and lower analog comparators, an all digital implementation isalso possible by sampling and converting into digital form the feedbackvoltage Vfb, and then performing the low pass filtering, offset, andcomparison functions in the digital domain. The description is thus tobe regarded as illustrative instead of limiting.

What is claimed is:
 1. A multi-phase switch mode power supply circuitfor controlling the transfer of power to a power supply output through aplurality of phase inductors, and for regulating voltage at the powersupply output, the circuit comprising: a switch mode power supply (SMPS)controller portion having one or more phase control outputs that are tobe coupled to one or more control inputs of one or more switch circuits,respectively, that conduct inductor current through one or more otherphase inductors, from amongst the plurality of phase inductors, whereinthe controller is configured to pulse the phase control outputs so as toregulate voltage at the power supply output responsive to a feedbackvoltage from the power supply output; and a transient mode portionhaving a phase control output to be coupled to one or more controlinputs of one or more switch circuits that conduct inductor currentthrough one or more transient phase inductors, from amongst theplurality of phase inductors, and a slew mode control circuit configuredto detect a high slope and then a low slope in the feedback voltage and,in between detection of the high slope and the low slope, pulse thephase control output of the transient mode portion so that the switchcircuit that conducts transient phase inductor current adds power to, orsinks power from, the power supply output.
 2. The power supply circuitof claim 1 wherein the slew mode control circuit is configured to: causethe phase control output of the transient mode portion to be i) assertedto turn on the switch circuit that conducts transient phase inductorcurrent, in response to both the high slope being detected and detectingthat absolute value or level of the feedback voltage is below apredetermined threshold, and ii) de-asserted to turn off the switchcircuit in response to the low slope being detected.
 3. The power supplycircuit of claim 2 wherein each of the one or more transient phaseinductors has smaller inductance than every one of the other phaseinductors.
 4. The power supply circuit of claim 1 wherein the slew modecontrol circuit further comprises: a further phase control output; and anon-overlapping control driver circuit having a first input coupled tothe phase control output, a second input coupled to the further phasecontrol output, and first and second outputs to be coupled to control ahigh side transistor and a low side transistor, respectively, of theswitch circuit.
 5. The power supply circuit of claim 4 wherein thetransient mode portion is to control droop in the power supply outputvoltage, by being configured to a) assert the phase control output so asto turn ON the high side transistor and turn off the low side transistorthrough the driver circuit, to increase the inductor current, inresponse to detecting the high slope in the feedback voltage, and thenb) de-assert the phase control output so as to turn off the high sidetransistor and turn ON the low side transistor through the drivercircuit, to decrease the inductor current towards zero, in response todetecting the low slope in the feedback voltage, and then c) assert thefurther phase control output to turn off both the low side transistorand the high side transistor in response to a) the inductor currentthrough the transient phase inductor reaching zero, and b) the feedbackvoltage having reached the low slope or its slope has changed polarity.6. The power supply circuit of any claim 1 wherein the transient modeportion comprises a current limit-based blocker circuit that isconfigured to govern pulse width at the phase control output, based onan output voltage error-based controllable value that represents avariable limit on inductor current through said one or more transientphase inductors that is clamped to a lower limit and an upper limit, andthat has a programmable value which represents a programmable thresholdfor the feedback voltage.
 7. The power supply circuit of claim 6 whereinthe transient mode portion comprises a further programmable value thatrepresents off-time of the phase control output.
 8. The power supplycircuit of claim 1, wherein the transient mode portion comprises an ontime-based blocker circuit that is configured to set pulse width of thephase control output based on an output voltage error based controllablevalue that represents a time interval that is clamped between a shortlimit and a long limit, and that uses a programmable value thatrepresents a programmable threshold for the feedback voltage.
 9. Thepower supply circuit of claim 8 wherein the transient mode portioncomprises a further programmable value that represents off-time of thephase control output.
 10. The power supply circuit of claim 1 whereineach of the one or more transient phase inductors has at least ten timessmaller inductance than every one of the other phase inductors.
 11. Thepower supply circuit of claim 1 wherein each of the one or moretransient phase inductors has at least one hundred times smallerinductance than the other phase inductor that has largest inductance.12. The power supply circuit of claim 1 further comprising: aprogrammable circuit configured to store first and second parametersthat define the high and low slopes, respectively; and a digitalcommunication interface to the programmable circuit, through which thefirst and second slope parameters can be written by a device external tothe power supply circuit.
 13. The power supply circuit of claim 1wherein the slew mode control circuit comprises: a first analogcomparator whose output is a first bistable comparison indicationbetween a first low pass filtered and offset version of the feedbackvoltage and a second low pass filtered version of the feedback voltage;a second analog comparator whose output is a second bistable comparisonindication between the second low pass filtered version of the feedbackvoltage and a third offset version of the feedback voltage; and logiccircuitry that causes the phase control output to be a function of thefirst and second comparison indications.
 14. The power supply circuit ofclaim 13 wherein the first low pass filtered and offset version isproduced using a first low pass filter, and the second low pass filteredversion is produced using a second low pass filter that has a lower cutoff frequency than the first low pass filter.
 15. The power supplycircuit of claim 13 wherein the first and second low pass filters havevariable cut off frequencies that are set as per the first and secondparameters which are stored in the programmable circuit.
 16. The powersupply circuit of claim 1 further comprising a programmable circuitconfigured to store a transient mode value or a steady state mode value;and a digital communication interface to the programmable circuit,through which the transient and steady state mode values can be writtenby a device external to the power supply circuit, wherein the transientmode portion is automatically self-configured in accordance with theprogrammable circuit, into i) its phase control output remaining part ofthe transient mode portion and being enabled only during transient loadconditions, when the transient mode value has been written, and ii) itsphase control output being re-configured to be part of a steady statecontrol loop and being enabled during both transient load conditions andsteady state conditions, when the steady state mode value has beenwritten.
 17. The power supply circuit of claim 1 wherein the switchcircuits that conduct the inductor currents through the one or moreother phase inductors and the one or more transient phase inductorscomprise a plurality of switching transistors that conduct the inductorcurrents and are to receive the same power supply input voltage.
 18. Thepower supply circuit of claim 1 wherein the switch circuits that conductthe inductor currents through the one or more other phase inductors andthe one or more transient phase inductors comprise a plurality ofswitching transistors that conduct the inductor currents and are ratedto operate at 5 Volts.
 19. The power supply circuit of claim 1 whereinthe switch circuits that conduct the inductor currents through the oneor more other phase inductors and the one or more transient phaseinductors comprise a plurality of switching transistors that are formedon the same integrated circuit die as the SMPS controller portion andtransient mode portions.
 20. The power supply circuit of claim 1 incombination with the plurality of phase inductors, which include the oneor more transient phase inductors and the one or more other phaseinductors, wherein the other phase inductor is a discrete, multi-loopinductor and the transient phase inductor is a partial loop of wire thatis located close to the discrete, multi-loop inductor so as to bemagnetically coupled therewith.
 21. A method for DC-DC power conversionthat uses a multi-phase switch mode power supply circuit, comprising:during a steady state load condition on a power supply output,controlling one or more switch circuits that transfer power to the powersupply output through one or more other phase inductors, respectively,that are a subset of a plurality of phase inductors, and to regulatevoltage at the power supply output; and during a transient loadcondition on the power supply output, controlling one or more switchcircuits to transfer power to the power supply output through one ormore transient phase inductors, respectively, that are a differentsubset of the plurality of phase inductors, and wherein the one or moreswitch circuits are controlled so that the one or more transient phaseinductors do not transfer power to the power supply output during thesteady state load condition, and wherein during the transient loadcondition i) the switch circuit for the transient phase inductor ispulsed so as to increase the inductor current, when slope of the powersupply becomes high, and then ii) the switch circuit is turned off suchthat the inductor current decreases to zero, when the slope of the powersupply output becomes low, and iii) i) and ii) are repeated, wheneverthe slope of the power supply output becomes high and the inductorcurrent reaches zero in ii).
 22. The method of claim 21 wherein each ofthe one or more transient phase inductors has smaller inductance thanevery one of the other phase inductors.
 23. The method of claim 21wherein the transient load condition is voltage droop, and a high slopeis more negative, or more inclined downward versus time, than a lowslope.
 24. The method of claim 21 wherein the low slope can be negativeor inclined downward versus time, or it can be positive or inclinedupward versus time.
 25. The method of claim 21 wherein the transientload condition is voltage overshoot, and a high slope is more positive,or more inclined upward versus time, than a low slope.
 26. The method ofclaim 25 wherein the low slope can be positive or inclined upward versustime, or it can be negative or inclined downward versus time.
 27. Themethod of claim 21 further comprising: resetting a watch dog timer inaccordance with a control signal that is controlling turn on of a switchcircuit for a phase other than the transient phases; and in response tothe watch dog timer expiring, forcing the switch circuit for the phaseother than the transient phases to turn on and forcing the switchcircuits for the transient phases to turn off.